MOLLUSCA — GASTROPODS 247 



and Tertiary of North America, such as T. mortoni of the 

 Eocene of the Atlantic and Gulf coastal plain. 



The terminals of various genetic lines of Turritella are found 

 in the species of the genus Vermicularia (Tertiary to the present) . 

 In this genus, the earlier, younger, whorls are like Turritella, 

 while the later, adult, whorls are uncoiled and somewhat twisted. 



1. Sketch {a) side view showing the elongate spire and the 

 aperture; {b) side view of an internal mold. Note spire, 

 whorls, aperture. 



2. How much of the shell did the body of the animal occupy ? 



3. How does this compare with the portion of the shell filled 

 by the internal mold ? n 



4. How is an internal mold formed? 



5. In what kind of rocks are the best internal molds found ? 



6. What information about the soft body of the animal does 

 this form of preservation give ? 



7. Show that Vermicularia is descended from Turritella. 



8. Given only the irregularly coiled portion of Vermicularia, 

 show that it was secreted by a gastropod and is not {a) a worm- 

 tube like Serpula nor {b) a siphonal tube like that of Teredo. 



9. Why is Turritella placed in the class Gastropoda ? 



10. What according to derivation does the word gastropod 

 mean ? Why applied to the members of this class ? 



1 1 . How is the gastropod shell built ? 



12. When the animal moves which way does the spire point ? 



Clio (Fig. 108). Tertiary to present. 



Clio is a tiny gastropod, often shaped like a narrow, elongate 

 cone, about an inch long. It occurs in immense swarms in 

 northern and southern seas, swimming in the open ocean by 

 flapping movements of the fin-like lobes of its foot. It forms 

 much of the food of whales. 



It differs from Busycon in its symmetrical, uncoiled body 

 and shell, and in the expansion of the foot into the two sym- 

 metrical fins used in swimming. 



The shell is thin, in the form of a tapering tube, without trans- 

 verse septa. From its larger and open end project the two large 

 wing-like lobes of the foot. This open end is anterior ; there 



